A device which has been found useful in the reduction of undesirable emissions from the exhaust of internal combustion engines is the catalytic converter. It has been found that a catalytic device with a noble metal catalyst such as platinum or palladium, when supplied with an exhaust gas mixture containing oxygen and hydrocarbon fuel in a ratio maintained within a narrow "window" around stoichiometry, shows high efficiency in simultaneously oxidizing carbon monoxide and unburned hydrocarbons and reducing oxides of nitrogen. However, the only way to maintain air-fuel ratio within such narrow limits is with a closed loop control system, many of which have been proposed.
One way of accomplishing such a closed loop system is to provide a zirconia sensor in the engine exhaust system with appropriate signal processing electronics to control air-fuel ratio adjustment means in a standard engine carburetor. This approach has at least one attractive feature in that the feedback control elements can be basically added on to existing engines, which fact facilitates adoption of the method and may minimize the additional cost.
An engine with standard carburetor and zirconia exhaust sensor, however, has certain modes of operation in which closed loop control is not practical. These include closed throttle operation or idling, wherein a constant ratio idle jet rather than the controlled main jet supplies fuel to the engine, and cold start operation, since a zirconia sensor does not produce a useful output signal until raised to a minimum operating temperature. In addition, as is the case in many proposed systems, if engine induction vacuum is used to transmit the signal at some point in the feedback loop, this vacuum will fall to an unusable level during wide-open throttle engine operation. Thus open loop fuel control, with possible higher emissions, is necessary during these modes of engine operation.
Over any total period of engine use, a substantial portion of that period will be spent in the normal operating mode where closed loop control is practical and low emissions are obtained. Some portion of this period, however, will be spent in one of the modes in which open loop control is necessary and the total average engine emissions will thus depend partly upon the level of open loop engine emissions.